1. Field of the Invention
This invention relates to sliding vane positive displacement pumps, and more particularly, to such pumps having hydraulically actuated vanes.
2. Description of the Prior Art
One of the primary concerns in using positive displacement pumps of the sliding vane type is maintaining the vanes in contact with the inner surface of a liner along which the vanes ideally move in transporting liquids through the pump. As is known to those skilled in the art, there is a natural centrifugal force acting outwardly on the vanes as the rotor rotates. However, in most applications involving liquids, the vanes will move radially inwardly away from the inner surface at certain points and thus fluid will slip by the vanes. The result is a decrease in pumping efficiency. Many means have been devised to place additional outwardly acting forces on the vanes in addition to the centrifugal force so that the vanes will track the inner surface of the liner properly for maintaining higher pumping efficiency.
In many cases, holes are drilled through the rotor and shaft interconnecting opposite pairs of slots in the rotor. Pins or other actuators are then slidingly positioned in the holes such that as one vane moves inwardly, the opposite vane is forced outwardly. One problem with such systems is that the dimension across the liner may not be constant. If this is the case, the pin must be somewhat shorter than the normal distance between the two innermost edges of the opposite vanes. Even if the distance across the liner is constant, the solid pin arrangement allows no compensation for wear on the outer edges of the vanes or wear by the contact of the vanes by the pin. In either case, the result is that the pin has some movement between the vanes and is not maintained in constant contact with the inner edges thereof. The pin is thus bounced back and forth between the vanes as the rotor turns. In other words, the pin impacts the inner surface of each of the opposite vanes for each revolution of the rotor. At the high speeds with which positive displacement pumps may be operated, the force of impact of the pin on the vanes can be quite high, quickly resulting in damage to the vanes. One solution has been the use of hard metal bumpers attached to, or molded into, the vanes to protect the inner surface. The result is an expensive vane, and the pins will eventually wear the bumpers as well.
Another method of providing outward force on vanes for sliding vane pumps is the use of a coil spring positioned between the inner surface of the vane and the rotor slot. One such arrangement is shown in U.S. Pat. No. 2,541,405 to Chapman, in which a hole is countersunk in each vane to contain and guide the spring. There are many variations on the spring actuated vane arrangement. The system has the advantage of compensating for wear on the outer surface of the vanes, but a frequent problem is wear on the outer surfaces of the spring.
Another solution along similar lines has been the use of two pins disposed in a hole intercommunicating opposite slots with a spring positioned therebetween to help absorb shock and compensate for wear. This arrangement contains the spring sufficiently so that it will not skew, but still has the disadvantage of wear on the outer surface of the spring because the spring must slide in the hole in the rotor.
All of these mechanically actuated vanes work with varying degrees of success, at least when the fluid being pumped provides some lubrication for the various moving components. However, mechanical actuation has been shown to be particularly unsatisfactory in cases where low viscosity fluids such as propane, anhydrous ammonia, and other light hydrocarbons are being pumped. Because such fluids provide virtually no lubrication, mechanical actuation systems wear quite rapidly, resulting in undesirably short service life.
In an effort to eliminate the problems of mechanical actuation systems, hydraulic vane actuation systems have been developed. Such a pump is disclosed in U.S. Pat. No. 3,072,066 to Kennedy, et al., assigned to the assignee of the present invention. In this pump, a channel in a sideplate adjacent the rotor directs fluid beneath the vanes to provide hydraulic force to move the vanes radially outwardly. A variation on such a system, but not disclosed in Kennedy et al., includes holes drilled in the rotor which direct fluid beneath the vanes.
These hydraulic vane actuating systems have proved successful for many years in the pumping of low viscosity fluids. However, even with such systems, the actuation of the vanes may become erratic under certain conditions such that the vane will not properly track along the liner and the pump will correspondingly lose efficiency. The vane used in the pump of the present invention includes flow passageways in the vane itself which direct fluid from the leading edge of the vane to the radially inner edge to assist in forcing the vane outwardly to the liner adjacent the pump inlet. The passageways through the vanes intersect the radially outer edge and a beveled leading edge which faces generally in the direction of rotation of the rotor. Further, the passageways provide fluid relief as the vanes are forced inwardly by the liner adjacent the pump outlet.
U.S. Pat. No. 2,982,223 to Rosaen discloses a vane with a beveled outer edge which generally faces in the direction of rotation. The vane has a passage which opens to the forward side of the vane. U.S. Pat. No. 4,521,167 to Cavalleri, et al. shows a vane with a somewhat tapered leading outer edge with radial slots in the leading side of the vane. Neither of these vanes has a distinct radially outer edge. Instead, they have rounded corners contacting the inner surface of the liner as the rotor rotates. In the vane of the present invention, a distinct outer edge is provided, and the passageways through the vane intersect this edge as well as the beveled leading edge. Testing has shown that intersection of the passageways with both the leading and outer edges apparently helps prevent detrimental effects of the fluid boundary along the inner surface of the liner as the outer edge of the vane travels thereacross.
In addition to eliminating the problems associated with mechanical vane actuating systems, the hydraulic vane actuating system of the pump of the present invention also eliminates problems with previous hydraulic actuation systems. Thus, the pump and vane of the present invention particularly provide a better system for use with low viscosity fluids, representing a distinct improvement over the prior art.